From 03144b5869d2921dafbea477246aeb5d4eb5fc38 Mon Sep 17 00:00:00 2001
From: Michael Dalton <mwdalton@google.com>
Date: Thu, 16 Jan 2014 22:23:29 -0800
Subject: [PATCH] lib: Ensure EWMA does not store wrong intermediate values

To ensure ewma_read() without a lock returns a valid but possibly
out of date average, modify ewma_add() by using ACCESS_ONCE to prevent
intermediate wrong values from being written to avg->internal.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Michael Dalton <mwdalton@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 lib/average.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/lib/average.c b/lib/average.c
index 99a67e662b3c..114d1beae0c7 100644
--- a/lib/average.c
+++ b/lib/average.c
@@ -53,8 +53,10 @@ EXPORT_SYMBOL(ewma_init);
  */
 struct ewma *ewma_add(struct ewma *avg, unsigned long val)
 {
-	avg->internal = avg->internal  ?
-		(((avg->internal << avg->weight) - avg->internal) +
+	unsigned long internal = ACCESS_ONCE(avg->internal);
+
+	ACCESS_ONCE(avg->internal) = internal ?
+		(((internal << avg->weight) - internal) +
 			(val << avg->factor)) >> avg->weight :
 		(val << avg->factor);
 	return avg;
-- 
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